Early aircraft were controlled by either warping the wings or by moving separate small control surfaces directly through cables or linkage directly connected to the pilot's control stick. Large control surfaces were sometimes “aerodynamically balanced” by means of a portion of the control surface panel being extended forward of the hinge point on the control surface. The increased speed and size of later developed aircraft caused the control surface loads to become sufficiently large that the effort available from the pilot was not sufficient to control the aircraft. This in turn led to the development of various types of hydraulic and electric power arrangements to move the larger control surfaces.
In general, the forces on an aircraft control surface panel are caused by the deflection of the panel into an air stream. The pressure of the moving air stream against the surface of the control panel results in a “moment” about the control panel hinge that must be provided for by the control panel actuator. This moment is often called the “hinge moment” or the “aerodynamic moment.” This moment is generally proportional to the deflection angle of the control panel for a given flight speed. However, for a given deflection angle of the control panel into the air stream, the moment on the panel is a function of the square of the aircraft speed. Thus, a panel actuation system may be designed for a load at a relatively low speed and high deflection (e.g., 30°); but when operating at a higher speed, the actuation system may experience the same load at a much lower deflection angle of the control panel (e.g., 10°).
Referring to FIG. 1a, a control surface and simple actuator is illustrated to help understand the power needed to deflect the control panel. The power required to deflect the control panel is a function of the hinge moment (Γ) and the rate of motion of the panel into the air stream. The hinge moment is proportional to the deflection angle (Θ), although not necessarily in a linear relationship as illustrated in the graph of FIG. 1b. 
When the control panel is being deflected into the air stream, the hinge moment and power are supplied by the actuator. In this instance work is being done on the control panel. When the control panel is returned to the neutral position (e.g., zero deflection angle), the actuator restrains and controls the control panel, and work is done by the control panel on the actuator. The actuator system has no means to recover this “aiding load” energy when the control panel is returned to the neutral position. Furthermore, in the case of an hydraulic actuation system, not only is the “aiding load” energy not recovered, but also additional energy is required from the hydraulic power system to fill a low pressure side of the actuator when the control panel is returned to the neutral position. With a conventional hydraulic actuator which is designed to provide the maximum anticipated hinge moment, the power consumed by the actuator is only a function of the rate of motion and the maximum design moment, not the prevailing hinge moment or the direction of motion.
Since the external loads (i.e., the air stream) on the control panel always tend to act in a direction to restore the control panel to its neutral position, it would be highly desirable to provide some system and method for recovering and storing the energy that is effectively imparted to the control panel by the air stream when the control panel is allowed to return to its neutral position. It would also be highly useful if such a system and method could be employed to assist in deploying the control panel back into the air stream when deployment of the control panel is required. Such a system and method would not only recover the “aiding load” energy that is presently wasted, but the hinge moment provided by such a system and method could be used to provide a portion of the maximum design hinge moment during any subsequent deployment of the control panel. This would allow the actuator used with the control panel to be reduced in size, and would thus reduce the power needing to be delivered to the actuator(s) at any motion and angular speed of its associated control panel.